Cancers, or malignant tumors which are characterized by continuous cell proliferation and cell death, can be classified into three categories: carcinomas, sarcomas, and leukemia. Recent reports show that approximately one in eight women contracts breast cancer and that the risk of prostate cancer is about 9.5% among men over 50 years of age (Helzlsouer, K. J. (1994) Curr. Opin. Oncol. 6: 541-548; Harris, J. R. et al. (1992) N. Engl. J. Med. 327:319-328). Cancer cells have been shown to exhibit unique gene expression, and dozens of cancer-specific genetic markers, tumor antigens, have been identified.
Tumor antigens are surface molecules that are differentially expressed in tumor cells relative to non-tumor tissues. Tumor antigens make tumor cells immunologically distinct from normal cells and provide diagnostic and therapeutic targets for human cancers. Several monoclonal antibodies have been identified which react specifically with cancerous cells such as T-cell acute lymphoblastic leukemia and neuroblastoma (Minegishi, M. et al. (1989) Leukemia Res. 13:43-51; Takagi, S. et al. (1995) nt. J. Cancer 61: 706-715). In addition the discovery of high level expression of the HER2 gene in breast tumors has led to the development of therapeutic treatments (Liu, E. et al. (1992) Oncogene 7: 1027-1032; Kern, J. A. (1993) Am. J. Respir. Cell Mol. Biol. 9:448-454).
Tumor antigens have been characterized either as membrane proteins or as altered carbohydrate molecules of glycoproteins or glycolipids on the cell surface. A multigene family encoding type III integral membrane proteins which traverse the cell membrane four times has been identified (Wright, M. D. and Tomlinson, M. G. (1994) Immunol. Today 15:588-94). The transmembrane 4 superfamily (TM4SF) proteins are found predominantly in cells of hematopoietic origin and in tumors and include a number of platelet and endothelial cell membrane proteins, CD9 (lung adenocarcinoma antigen MRP-1), the platelet and melanoma-associated antigen CD63, leukocyte surface glycoproteins, CD53, CD37, CD63, and R2, the tumor associated antigen TAPA-1 (CD81), the colon carcinoma antigen CO-029, mink lung epithelial protein TI-1, and the tumor-associated antigens, L6 and SAS, a gene amplified in human sarcomas (Wright and Tomlinson, supra; Jankowski, S. A. et al. (1994) Oncogene 9:1205-11). These proteins share 25-30% amino acid sequence identity.
In the TM4SF proteins, the N- and C-termini are intracellular, and the major hydrophilic domain, located between transmembrane domains 3 and 4, is extracellular. TM4SF proteins are most conserved in their transmembrane and cytoplasmic domains and most divergent in their hydrophilic extracellular domains which contain N-linked glycosylation sites. The high level of conservation in the transmembrane and cytoplasmic domains suggests an effector/signaling function. The divergence of the extracellular domains suggests that these hydrophilic domains provide functions specific to each protein such as ligand binding or protein-protein interaction (Wright and Tomlinson, supra).
A number of TM4SF proteins have been implicated in signal transduction, control of cell adhesion, and regulation of cell growth and proliferation (Wright and Tomlinson, supra; Jankowski, supra). Expression of some TM4SF proteins is associated with a variety of tumors and is altered when cells are activated or dividing. Other TM4SF proteins are implicated in cell growth due to their association with tumor cells. For example, CD9, CD53, and CD82 are upregulated when lymphocytes are activated while the expression of CD37 is abolished when B cells are activated. Although CD9 is not expressed on resting B and T lymphocytes, it is a marker for 90% of non-T acute lymphoblastic leukemia cells and for 50% of acute myeloid and chronic lymphoid leukemias. Anti-CD9 antibodies inhibit the motility of a variety of cancer cell lines and inhibit the metastatic potential of the mouse BL6 cell line (Miyake, M. and Hakomori, S. (1991) Biochem. 30:3328-34). Similarly, CD63 is not expressed on normal tissue melanocytes, but it is expressed in early stage melanoma. Another member of the TM4SF superfamily, the L6 surface antigen, is differentially expressed on lung, breast, colon, and ovarian carcinomas (Marken, J. S. et al. (1992) Proc. Natl. Acad. Sci. USA 89: 3503-3507; Marken, J. S. et al. (1994) J. Biol. Chem. 269: 7397-7401).
The discovery of a tumor-associated antigen, similar to the tumor-associated L6 antigen, and the polynucleotides which encode it satisfies a need in the art by providing new compositions which are useful in diagnosing, preventing, and treating inflammation and disorders associated with cell proliferation.